1. Field of the Invention
This invention relates to a process for stabilizing the idle speed of an internal combustion engine with controlled ignition for a motor vehicle.
2. Description of the Prior Art
It is known how to regulate the idling speed of an internal combustion engine by keeping it the idling speed as close as possible to a predetermined value and by bringing it back to the predetermine value quickly when it deviates therefrom under the influence of an outside disturbance (variation of richness, advance, letting up on the accelerator, pulling of the power steering, etc.). For this purpose, appropriate means are used to affect the amount of air admitted into the intake manifold, as described, for example, in French patent application No. 8215193.
However, at idling, this type of engine exhibits a pumping phenomena due to poor combustion that is reflected by sudden significant variations of the idling speed. The constraints associated with the measures taken to combat pollution and promote energy savings have further aggravated this problem by calling for operating richnesses close to the equivalence ratio which give rise to disturbing phenomena (divergence from the flame propagation speeds, gaps in richness due to oxygen probes, etc.).
As a result, there can result engine speed oscillations of such a large amplitude that a regulation of idling speed by automatic control at a predetermined speed is not able to eliminate oscillations.
On the other hand, it is known how to make the angle of ignition advance tend to stabilize the idling speed and to eliminate the above noted oscillations. FIG. 1 shows a standard law of ignition advance as a function of the speed according to which the advance is constant over the evolution range of the idling speed. An improvement represented in FIG. 2 consists in providing a law of advance having a negative slope on the evolution range of the idling speed, any increase of the idling speed tending to be compensated by a decrease in the advance. However, at a speed slightly greater than that of the nominal idle (increased idling when cold, for example), there is a highly destabilizing positive advance slope since the variations of speed tend to be amplified.